Microsensors are miniaturized sensing devices that are increasingly being employed for sensing dynamic motion such as acceleration and rate of change of position. Accelerometer microsensors measure the second derivative of displacement with respect to time and include linear and angular accelerometer microsensors. Linear and angular accelerometers are frequently employed to generate an output signal (e.g., voltage) proportional to the sensed acceleration for use in vehicle controls systems. For example, the sensed output from a linear accelerometer microsensor may be used to control safety-related devices on an automotive vehicle, such as front and side impact air bags, or may be employed for vehicle dynamics control and suspension control applications. The sensed output from an angular accelerometer microsensor may be employed to determine a potential vehicle rollover event, to control various automotive control devices, and to control disc drive read/write head assemblies.
Many microsensors are capacitive type sensing devices that employ a capacitive coupling between a fixed plate and a movable plate that is movable in response to the sensed motion. One example of a linear accelerometer microsensor is disclosed in application Ser. No. 10/059,010, filed Jan. 31, 2002, and now U.S. Pat. No. 6,761,070, issued on 13 Jul. 2004, entitled “MICROFABRICATED LINEAR ACCELEROMETER”, assigned to the assignee of the present invention, which is hereby incorporated herein by reference. An example of an angular accelerometer microsensor is disclosed in U.S. Pat. No. 6,393,914, issued on 28 May 2002, entitled “ANGULAR ACCELEROMETER”, assigned to the assignee of the present invention, which is hereby incorporated herein by reference. Other examples of angular accelerometer microsensors disclosed in commonly assigned U.S. application Ser. No. 10/055,536, filed Feb. 28, 2002, and now U.S. Pat. No. 6,718,826 issued on 13 Apr. 2004, entitled “BALANCED ANGULAR ACCELEROMETER”, and application Ser. No. 10/085,793, filed Feb. 28, 2002, and now U.S. Pat. No. 6,666,092, issued on 23 Dec. 2003, entitled “ANGULAR ACCELEROMETER HAVING BALANCED INERTIA MASS”, both of which are also assigned to the assignee of the present invention, and are hereby incorporated herein by reference.
The aforementioned microsensors are generally fabricated by employing micro-electro-mechanical (MEM) fabrication techniques, such as etching and micromachining processes. Following manufacture of the microsensor, the microsensor is typically tested to determine if the microsensor functions properly and to determine the need for any scale factor calibration. In the past, microsensors were typically tested by employing expensive hardware including a mechanical shaker designed to physically shake the microsensor under test to apply a predetermined motion (e.g., acceleration) to the microsensor. In response to applying the predetermined motion, the microsensor output is monitored and compared to an expected value. The deviation between the expected and measured values is processed to determine any error. The error may be used to determine if the microsensor is faulty and/or to trim integrated circuitry to adjust the scale factor calibration of the microsensor, prior to using the microsensor. While the mechanical shaker testing approach offers the ability to test the microsensor immediately following manufacture, the test procedure generally cannot be easily implemented once the microsensor is employed in an application. Additionally, the mechanical hardware of the shaker is generally expensive.
It is therefore desirable to provide for a low cost, easy to implement test circuit for testing the functioning of a microsensor. It is further desirable to provide for a reliable self-test circuit for testing the microsensor that allows for testing while the microsensor is implemented in its intended application.